To you, those birds might just sound like nature in the background. But to the trained ear, that call can mean the difference between life and death.

It gives wildlife early warning and saves them from trouble.

Think about when you talk to your friend over the phone. Without seeing their face, you can tell their mood by their voice or intonation. The same goes for birds, and learning their language can tell you about your surroundings.

When you don't hear companion calls, when you don't hear a song, and the whole place gets silent, that's an alarm.

This practice, which can be leveraged in the wild to hunt for meals or avoid nearby predators, was mastered by Justin Alexander, a man whose life appeared torn from the pages of an adventure novel.

I'm at the top of Throng La Pas in the Nepal, 5,600 something meters, and I'm in flip flops.

His Instagram captured moments of spear phishing with remote island tribes, crossing deserts on motorcycle, and rebuilding schools in Nepal.

You could drop Justin off in the middle of nowhere with nothing, you'd be okay.

Yet in 2016, under suspicious circumstances, Justin Alexander disappeared.

He was making it seem like that was going to be his last trip.

I said to him, Are you sure you want to do this, man?

If Justin were going to write a story about himself, wouldn't he write it this way?

How could a train survivalist and world traveler simply vanish? Find out on the new podcast from Tenderfoot TV. Status Untraced, Coming April 17th.